1. Field of the Invention
The present invention relates generally to electronic ballasts for fluorescent lamps and relates more particularly to a simplified hybrid ballast control and package.
2. Description of Related Art
Electronic ballasts are used in a number of fluorescent lighting applications, particularly those that operate with a switching half-bridge. Such an electronic ballast is illustrated in U.S. Pat. No. 6,008,593 to International Rectifier Corporation. Electronic ballast controls have evolved to include a wide range of functionality and features including power factor correction and fault detection in response circuitry. A typical electronic ballast includes several control ICs that operate switches in the electronic ballast and receive feedback signals to control the fluorescent lamp driven by the switching half-bridge. A control IC for controlling the switching half-bridge typically includes an oscillator that is used to drive the switching signals for the half-bridge switches coupled to the fluorescent lamp load. One type of implementation of an electronic ballast using an oscillator involves connecting a voltage controlled oscillator (VCO) into the electronic ballast and driving the VCO with an appropriate signal to modify the switching frequency as desired. For example, in the case of fluorescent lamp applications, the switching frequency of the electronic ballast can be adjusted to vary during startup or under fault conditions to improve operation of the electronic ballast.
The electronic ballast also typically fulfills various control design features such as preheating lamp filaments, igniting the lamp, driving the lamp to a desired power level, detecting lamp fault conditions and safely deactivating the electronic ballast. The use of electronic ballasts for gas discharge lamps such as fluorescent lamps are widely available due to the use of power MOSFET switching devices and insulated gate bipolar transistors (IGBT) that replace previously used power bipolar switching devices. Monolithic gate driver circuits, such as the IR2155 sold by International Rectifier Corporation and described in U.S. Pat. No. 5,545,955 have been used for driving power MOSFETs or IGBTs in electronic ballasts. The IR2155 gate driver IC is advantageous in a number of applications because it is packaged in a conventional DIP or SOIC package with a small profile and is usable in standard manufacturing processes.
In each of the above described electronic ballast circuits, the control ICs is realized separately from the power switches in the switching half-bridge or in other sections of the electronic ballast. In addition, the control ICs use external components, such as passive components, to obtain features such as programmable preheat time, minimum frequency and so forth. The external components can be determined based on application specific criteria by a designer to improve flexibility of the control IC. In many applications, however, it is desirable to obtain a simplified, application specific ballast control with low cost and easy manufacturability to implement a lighting control system.